In our previous studies we have demonstrated that alterations in the thyroid state induced neonatally in the rat, result in significant developmental abnormalities correlated, at the neurochemical level, with altered brain protein synthesis, altered composition of some specialized neural membranes (e.g. myelin, synaptic vesicles) and altered number and characteristics of thyroid hormone receptors in discrete brain regions and in liver. Our current studies have followed essentially three major directions, each a direct continuation of our original research: 1) to investigate the regulatory role of thyroid hormones on brain development and behavior by studying the effects of these hormones on number and characteristics of monoamine (NE, DA and 5-HT) receptors in discrete brain areas, at specific ages and in male and female rats; 2) to extend our studies from normal neural tissue in vivo to cultured brain tumor cells in order to establish whether thyroid hormones similarly regulate structural (e.g. microtubule polymerization) functional (e.g. ATPase activity) and receptor distribution in these tumor tissues. Furthermore by comparing neuroblastoma and glioma cells we may draw some conclusions as to the differential actions of thyroid hormones on neuronal and glial cells; finally 3) to study fatty acid composition and fluidity of biomembranes (especially mitochondrial membranes most susceptible to thyroid hormones) in the myocardium and the brain of adult rats (more suitable for methodologic reasons to these experiments than developing rats) to obtain a better understanding and a unified explanation for the multiple metabolic and functional actions of thyroid hormones in the myocardium, brain and possibly other tissues.